Active impedance line feed circuit

Abstract

A typical active impedance line feed circuit includes tip and ring amplifiers (20, 40) being controlled in response to signals at tip and ring voltage taps (6, 7) by a control circuit (60) to exhibit a.c. impedance and d.c. resistance characteristics for the purpose of supplying energizing current via tip and ring terminals (2, 3) for tip and ring leads of two wire communication line. In one example of the active impedance line feed circuit, improved operational tolerance of longitudinal interference and of a ground fault condition is provided by a d.c. amplifier (210a) being responsive to voltages at the tip and ring terminals. The circuit is characterized in that an inverting input of the d.c. amplifier is connected via resistors (212a, 313a) to the tip and ring voltage taps. In operation, the d.c. amplifier provides complete cancellation of all common mode signals at the tip and ring voltage taps. A compensation amplifier (235) is also connected between the tip and ring voltage taps and is responsive to the output of a control amplifier (231) for compensating for non-symmetrical current, the non-symmetrical current being introduced between the tip and the ring voltage taps by normal operation of the control circuit. Thereby, a threshold of ground fault current limiting action is substantially constant under all operating conditions. The d.c. amplifier is conveniently manufacturable in integrated circuit technology along with the remaining amplifiers in the active impedance line feed circuit.

What is claimed is: 
     
       1. An active impedance line feed circuit including tip and ring amplifiers being responsive to a control signal being proportional to currents in tip and ring feed resistors for supplying energizing current and a.c. information signals to a two wire communication line via tip and ring terminals connected to outputs of the tip and ring amplifiers by the tip and ring feed resistors respectively, tip and ring voltage dividers including tip and ring voltage taps respectively, a control circuit including a control amplifier for generating the control signal, the control amplifier having a differential input connected across the tip and ring voltage taps, and a d.c. amplifier being responsive to signals at the tip and ring terminals for attenuating longitudinal signals at the tip and ring voltage taps and for reducing fault current conduction during an occurrence of a ground fault condition on the two wire communication line, the active impedance line feed circuit being characterized in that: an inverting input of the d.c. amplifier is connected to the tip and ring voltage taps by a first pair of resistive elements being of similar ohmic values, and an output of the d.c. amplifier is also connected to the tip and ring voltage taps by a second pair of resistive elements being of similar ohmic values, whereby in operation the d.c. amplifier is subjected to attenuated electrical potential as compared with potentials at the tip and ring terminals, whereby the d.c. amplifier is conveniently manufacturable in integrated circuit technology along with the other amplifiers in the active impedance line feed circuit.   
     
     
       2. An active impedance line feed circuit as defined in claim 1 further characterized by a compensation circuit being connected between the tip and ring voltage taps and being responsive to the output of the control amplifier for compensating for a non-symmetrical current, said non-symmetrical current being introduced between the tip and ring voltage taps by normal operation of the control circuit, whereby a threshold of ground fault current limiting action controlled by the d.c. amplifier is substantially unaffected by variations in differential signals appearing at the tip and ring voltage taps. 
     
     
       3. An active impedance line feed circuit as defined in claim 2, wherein the output of the control amplifier is resistively connected to the ring voltage tap, the compensation circuit comprising: a first amplifier having an inverting input connected to a junction between first and second resistive elements being connected in series between the output of the control amplifier and an output of the first amplifier, the output of the first amplifier being connected to the tip voltage tap by a third resistive element.   
     
     
       4. An active impedance line feed circuit as defined in claim 2 wherein the output of the control amplifier is resistively connected to the ring voltage tap, the compensation circuit comprising: a first amplifier having an inverting input connected to a junction of first and second resistive elements being connected in series between the output of the control amplifier and an output of the first amplifier, third and fourth resistive elements being connected between the tip and ring voltage taps, and a fifth resistive element being connected in series between the output of the first amplifier and a junction of the third and fourth resistive elements.